Rapid progress in the field of electronics is ushering in an era of smart and adaptable devices in diverse applications such as wearables [1], e-textiles [2], smart surfaces [3], mobile devices [4], and epidermal [5] and implanted electronics [6]. Such portable devices require optimal energy storage and supply systems to function independently [7]. An additional challenge for devices operating in biological working environments is often the need to be miniaturized and compliant, necessitated by the soft and conformal nature of the biological milieu. This has led to research in flexible lithium-ion batteries, fuel cells, and supercapacitors [8, 9]. Among these, supercapacitors (SCs) are emerging as an important class of energy storage systems due to their fast charge-discharge rate, high power density, and long cycling life [10]. Flexible SCs can be used to power miniaturized devices having low power requirement. To date, there have been significant efforts to fabricate SCs in various geometries including planar [11], fiber, and wire-shape [12] to achieve properties such as ultra-flexibility, compactness, light weight, mechanical, and electrochemical stability.
A traditional supercapacitor is primarily composed of four components: active electrode, carrier substrate, gel electrolyte, and charge collectors. To date, various active electrode materials have been identified [7, 13-15] and are usually deposited or printed on inert, flexible carrier substrates [16]. Gel electrolytes are used as electrode separators and ion conductors [17]. The electrodes are usually interfaced with metallic conductors to and for transport of charges. Even though a wide range of materials have been identified for flexible SCs, material selection for energy storage devices for in vivo, subcutaneous or deep tissue implantable operations is challenging. Such systems necessitate additional properties such as biocompatibility, biodegradability, and often, sustainable, environmentally benign processing [18]. For instance, degradable devices that can be resorbed by the body would eliminate the need for additional extractive surgery. Therefore there has been interest in the use of natural biomaterials [19, 20]. The biopolymers can provide 3D structural support to the electroactive materials such as biohydrogels [19], provide a dielectric planar support such as paper-based devices [11], be pyrolyzed to form porous carbon electrodes [21], and form gel electrolytes [19].
Important performance parameters are capacitance, charge-discharge characteristics, capacitance retention over cycling, and the ratio of energy density to power density [22]. Capacitance values can be normalized over loaded active material (mass), area, or volume of the electrode. In the above mentioned systems, the improvement in capacitance performance is largely driven by combining electrode materials and enhancement of porosity to increase active area [13, 19, 23, 24].
However, an issue with conventional SCs is that they are too large to be used in vivo [16]. A recent approach to improve performance and integration with microfabricated devices involves optimizing packaging of the electrode itself. Instead of having two planar electrodes with sandwiched gel electrolyte, an interdigitated alternating microelectrode array is used [14, 25]. Performance improvement is achieved due to a shortened diffusion path without problems of electrode short-circuiting [26]. However, bio-supercapacitors that can be manufactured with a biocompatible fabrication process and which can be integrated with miniaturized devices, are needed.